In an image-forming apparatus, such as a copying machine utilizing a so-called Carlson process, electrophotographic photosensitive elements having single layer type or double layer type photosensitive layer containing functional materials are used. Generally, a charge generating material and a charge transfer material are used in a resin as a film-forming or binding material. Recently, these materials have been used increasingly due to the advantages that various materials can be selected, the freedom of functional design and excellent producibility.
Also, in electrophotographic photosensitive elements having the aforesaid photosensitive layer, generally it is the practice to form a surface protective layer containing a resin as a film-forming or binding material on the photosensitive layer for improving the abrasion resistance of the photosensitive layer.
Various resins have been used as the film-forming material for the photosensitive layer and surface protective layer. Of the known resins that are used, polyvinyl acetal is most suitable since the resin compound exhibits excellent dispersing properties for components such as the charge generating and charge forming materials, and excellent storage stability.
However, in photosensitive layers using polyvinyl acetal, a large amount of hydroxyl groups remain in the polyvinyl acetal and the photosensitive layer has a high hygroscopicity. Thus, this leads to problems for the photosensitive element as to resistance to surrounding conditions and the hydroxyl groups act as traps for charge carriers (positive holes) generated by light exposure. Also, the hydroxyl groups react with acid contained in the layer to form --OH.sub.2.sup.+ group, which form space charges which influences the counter potential of the photosensitive element, with the result that the sensitivity of the photosensitive element is lowered.
Similarly, in the case of surface protective layers containing polyvinyl acetal, a large amount of hydroxyl groups remain and the layer has a high hygroscopicity and reduced resistance to surrounding conditions. Also, there is a possibility that the mechanical strength of the protective layer is lowered by absorbing moisture and the adjacent photosensitive layer is deteriorated by the moisture passing through the surface protective layer.
It is known to use polyvinyl acetal dissolved in a solvent as a coating composition. Since polyvinyl acetal containing a large amount of hydroxyl groups, as described above, has a high solubility in an organic solvent, such as an alcohol, this leads to other problems in a lamination type photosensitive layer. For example, the polyvinyl acetal swells greatly or is dissolved by the organic solvent contained in the coating composition for the layer. This can occur when an upper layer is formed on a lower layer containing polyvinyl acetal, or when a surface protective layer is formed on an upper layer containing polyvinyl acetal of a lamination type photosensitive layer or on a single layer type photosensitive layer containing polyvinyl acetal. Thus, the interface between two layers becomes indistinct, which adversely influences the sensitivity characteristics of the photosensitive element. In particular, when the layer containing polyvinyl acetal is a surface protective layer, the strength of the surface protective layer is lowered.
Thus, it has been proposed to reduce the amount of hydroxyl groups remaining in the layer formed from polyvinyl acetal by adding an acetylacetone complex salt (metal acetylaceto-nate). The complex salt is hydrolyzed during drying of the coating composition causing a condensation reaction with the hydroxyl groups in the polyvinyl acetal in the coating composition. The acetylacetone complex salt is usually combined with the polyvinyl acetal in a solid state, such as a powder, etc., because of storage stability. However, since the acetylacetone complex salt has poor solubility in organic solvents such as alcohols, it requires lengthy stirring to uniformly dissolve the acetylacetone complex salt in a coating composition. Thus, there is the problem that it takes a long time to prepare the coating composition.
Also, when a large amount of the acetylacetone complex salt is added as a solid to the coating composition to further decrease the amount of hydroxyl groups remaining in the layer formed subsequently, it is difficult to dissolve the entire amount in the coating composition. This leads to the following problems.
First of all, an uneven coating is liable to occur when particles of the acetylacetone complex salt exist in the undissolved state in the coating composition. Additionally, the complex salt particles remain in the coated layer as foreign matter or as lengthwise stripes on the coated layer due to the movement of the particles on the coated layer in the coating direction. This results in a non-uniform coating layer and leads to defects in the images formed. Also, non-uniform distribution of acetylacetone complex salt in the coating layer forms portions containing a small amount of hydroxyl groups and portions containing a large amount of hydroxyl groups. In the case of a photosensitive layer, non-uniform distribution of the complex salt affects the sensitivity characteristics and the resistance to surrounding conditions. In the case of a surface protective layer, non-uniformity affects the resistance to surrounding conditions and the mechanical strength of the coating layer.
The present invention provides polyvinyl acetal coating compositions that can be formed into layers in an electrophotographic photosensitive element that obviate the above-noted problems.